The point is to not make a copy at all for the shared data of the same book. That's even what the page claims is happening (but it's wrong). That's the whole point of the flyweight pattern[1]. Instead, the example returns the same book instance for the same isbn in `createBook`, then blindly copies
the shared data like title, author, and isbn every time in `addBook`
to a new object.
> The shallow copy saves memory....
Versus not making a copy? A shallow copy of primitives still copies them and uses additional memory. Maybe there's some low-level optimization that makes it more efficient than that, but it's not relevant here. And there isn't any deep cloning happening in the example.
Might as well get rid of the complexity and instantiate a new Book class every time. And maybe stop shallow-copying class instances into new plain objects. It works for the example but has footguns.
Conveniently, there are other patterns on the site that would help you avoid creating an amalgamation of random merged class instances and objects.
The whole example is a mess.
[1]: https://en.wikipedia.org/wiki/Flyweight_pattern